Stereophonic amplifier system



June 2, 1964 R. sTARAl sTEREoPHoNIc AMPLIFIER SYSTEM United States Patent O 3,135,830 STEREOPHGNIC AMPLIFIER SYSTEM Rudolph Starai, Elmwood Park, Ill., assignor to Motorola, inc., Chicago, 1li., a corporation of Illinois Filed July 12, 1961, Ser. No. 123,576 4 Claims. (Cl. 179-1) This invention relates in general to stereophonic sound reproducing apparatus and in particular to such apparatus with an improved balance control for separate stereophonic sound channels.

Present day stereophonic sound reproducing devices include separate stereophonic channels, usually referred to as the right and left stereo channels, which individually translate diierent sound signals. For depth and realism, the separate sound signals are reproduced in two separate loudspeaker systems which are spaced apart to recreate the eect of hearing the sound pattern as that oi the original performance at multiple spaced locations. In such systems a control is usually provided so that the stereo channel may be balanced according to personal preferences and to compensate for diierent locations of the loudspeaker systems, or other ditierences in the amplifiers of the apparatus.

In the past, a problem was presented in maintaining desirable isolation between the separate stereo channels when a balance control is used. The control is usually a conventional potentiometer connected between the arnplitier channels and having a slidable tap arm connected to ground. However, such an arrangement may not provide optimum isolation or separation between the stereo channels and there may be a degree of cross coupling of the sound signals. This cross coupling results in an inferior reproduction fidelity in the separate stereo channels.

It is therefore an object of the present invention to provide stereophonic sound reproducing apparatus with a balance control and wherein improved separation is provided between the stereophonic channels in a simple, inexpensive and expeditious manner.

A feature of the present invention is the provision of stereophonic sound reproducing apparatus having a potentiorneter connected between separate stereo channels with a movable slider arm for balancing the same and a tixed tap at the electrical midpoint of the resistance element which is connected to ground thereby insuring optimum isolation between the stereo channels.

In the drawings:

FIG. l is a partial block and schematic diagram of stereophonic sound reproducing apparatus incorporating the present invention;

FIG. 2 is a series of diagrams useful in explaining the circuit ot FIG. 1; and

FIG. 3 is an exploded view of a potentiometer control for use in the circuit of FIG. l.

In practicing the invention, a stereophonic sound reproducing apparatus incorporates a balance potentiometer which is connected betwee the separate stereo amplifier channels. A slider arm is included as part of the potentiometer and is connected to ground such that movement of the slider arm will vary the relative outputs of the channels. A xed tap is also included at the electrical midpoint of the potentiometer resistance element and is likewise connected to ground potential. Movement of the slider arm will therefore vary the output of one channel while the output of the other channel remains constant due to the tixed tap. This action occurs on either side of the iixed tap. The xed tap makes Contact with the resistance element over a substantial area which effectively reduces the impedance in common between the stereo channels, thereby minimizing cross-coupling and maintaining optimum electrical separation therebetween.

Patented June 2, 1964 rice Referring now to FIG. 1, a stereophonic sound signaling apparatus is illustrated wherein separate stereo channels are included for translating and reproducing right and left stereophonic sound signals. The right and left stereophonic signals are derived from a disc type phonograph record 1t), by a dual channel audio signal source such as the stereophonic pickup cartridge 11. A left channel output lead therefrom supplies left channel audio intelligence to the input lead 12 of amplifier 22. Right channel audio intelligence is fed from the right channel output lead of cartridge 11 to the input lead 13 of right channel amplier 23.

In the respective input circuits of the stereo channel ampliiiers, a coupling capacitor 14 is `connected in series with a potentiometer 16 at the input of left channel amplilier 22 while a coupling capacitor 15 is connected in series with a potentiometer 17 at the input of right channel amplifier 23. The variable taps 18 and 19 of potentiometers 16 and 17 are ganged for operation as a single volume control to tap off input signals developed across the respective potentiometers and ground. Variable tap 18 is connected to the input grid of amplier tube 22 and variable tap 19 is connected to the iput grid of amplifier tube 23. Operating voltage is aplied to the anode of tube 22 through resistor 26. The cathode of tube 22 is connected to the reference potential through a cathode bias resistor 24 with a grid leak resistor 20 being connected between its control grid and the reference potential. Similiarly, the control grid of tube 23 is connected to variable tap 19 with resistor 21 forming the grid leak bias, resistor 27 supplying the operating voltage to its anode, and resistor 25 forming the cathode bias.

The output of left channel amplier 22 is applied through coupling capacitor 2S and resistor 30 to one side of stereo balance control 40 at junction 41. The output of right channel amplifier tube 23 is applied through coupling capacitor 29 and resistor 31 to the other side of balance control 40 at junction 42. Stereo balance control 40 includes both a fixed tap 44, located at its electrical midpoint, and a Variable tap 43. Taps 43 and 44 are both connected to ground potential. Left stereo signals are developed across the series resistor 30 and a resistance portion of balance control 40 connected in shunt to ground. Right stereo signals are developed across series resistor 31 and a resistance portion of balance control 40 also connected in shunt to ground. By varying the position of the variable tap 43, the value of the shunt resistance provided by potentiometer 40 will decrease the voltage drop across the series resistance in one channel and while maintaining the voltage drop across the series resistance in the other channel at a constant value due to the fixed tap 44. This action may occur on either side of the iixed tap 44. The change in voltage drop across series resistors 30 and 31 determines the level of signal applied to tubes 50 and 7 0 and thereby the amplification level of the stereo` phonic signal. The function of fixed tap 44 will be eX- plained in more detail further on.

Left channel stereo signals are applied from junction 41 through low frequency blocking capacitor 53 to one side of the variable resistor 54, forming a part of treble tone control 52. The arm of resistor 54 is connected to the control grid of left tone amplifier 50 with the other side of resistor 54 being bypassed to ground through capacitor 55. Variable treble control 52 controls the amount of high frequency components which are translated by tone amplifier 50.

The lower frequency components of the signal, which are not passed by capacitor 53 appear across network 51 which serves as a base control operating in a known manner. The capacitive elements in network 51 determine the amount of low frequency components which are developed across the resistive elements which are fed to the input of tone amplifier 50. Base control 51 is isolated from the treble control 52 by series resistor 57 and shunt resistor 56.

Cathode bias for left tone amplifier 50 is provided from the junction of the voltage divider network consisting `of resistors '62 and 63 connected between B-iand ground. A plate load resistor 64 is connected between the anode of amplifier 501and B|.

In a like manner, right stereo signals appearing at junction 42 are applied through low frequency blocking capacitor 73 and variable resistor 74 'to the input of right tone amplifier 7i). The right channel includes a base control 71 and a treble contro1'72 which lfunction in the same manner as described for the left channel. Base controls 51 'and 71 and treble controls v52 and 72 are respectively ganged together for operation as single controls. Cathode bias for right tone amplifier 76 is provided by the voltage divider network consisting of resistors 82 and 83 connected in seriesgbetween B+ and ground. Plate load resistor 84 is coupled between the anode of tube 70 and B+.

The output from left tone amplifier 50 is applied through coupling capacitor 65 to the left power amplifier 100 with the output from the right tone amplifier 70 being applied through coupling capacitor 85 to the right power amplifier 120. A low pass filter network 90 is connected across the left and right stereo channels whereby the low frequency stereo signals, up to approximately 300 cycles, are derived and fed to base power amplifier 110. The outputs from the power amplifiers are reproduced in separate loudspeakers, 100, V11i) and 120 respectively.

Returning now to a more detailed description of stereo balance control 40, it will be noted that both a fixed tap 44 and a variable tap 43 are included. FIG. 2 portrays a series of graphic illustrations useful in the 'explanation' of their purposes and function. InFIG. 2a, the resistive element of potentiometer 40 includes a series of parallel resistance paths, such as represented by Rx', Rx and Rx". Additional paths will also be present which run perpendicular to the aforementioned parallel paths. One such group of paths are represented as Ry and Ry, occurring at the point of contact of variable tap 43.

In electrical circuit terms, the arrangement can be represented by an equivalent T network as shown in FIG. 2b. It can be seen that the impedance represented by Ry provides a common coupling between terminals 41 and 42, thereby reducing the electrical separation of the circuits connected to these terminals and the terminal represented by variable tap 43. As mentioned previously, this cross couplingV between terminals 41 and 42 results in an inferior production fidelity in the separate stereo channels.

To reduce such undesirable cross coupling, a fixed tap 44 is incorporated to effectively reduce the value of cross-impedance Ry. The tap is located 'at electrical midpoint in the resistive element of potentiometer 4G and makes Contact over a relatively large area, as illustrated in FIG. 2c. By incorporating such a large Contact area for fixed tap 44, it can be readily seen thatV a substantial number of parallel cross-impedance paths will be provided which effectively reduces the value of the resultant cross-impedance Ry whereby optimum electrical separation is maintained between the stereophonic amplifying channels connected to terminals 41 and 42 respectively. Cross-coupling between the Yseparate stereophonic signals is thereby effectively prevented.

FIG. 3 illustrates the physical properties of potentiometer 40.V The general'construction of potentiometer 40 is effected in a known manner wherein a resistive element 132 is supported on'a fiat wafer 131 of insulating material. A rotatable shaft 130 extends through a center hole in the wafer 131 and receives thereon a rotating member 134 which includes a conductive spring member 135. Spring member 135 makes contact with resistive element 132 at a contact point depending upon the position of shaft 130. Electrical continuity is maintained from resistive element 132 through contacts 135a through contacts 135b to conductive washer 133 which is an integral part of terminal 43. As illustrated, an additional terminal is included which is connected to the approximate electrical midpoint ofresistive element 132, thereby providing a fixed tap 44. Mounting cover 136 encloses the internal component parts of potentiometer 40. An electrical switch 137 is further provided as'part of potentiometer 40 and VVoperates in a known manner;

The invention therefore provides an improved balance control for a stereophonic sound signaling translating apparatus wherein both a variable tap and a xed tap are included. The fixed tap is located at the electrical midpoint of the resistive element of the potentiometer and, when connected to ground, 'provides optimum isolation between the separate stereophonic 'channels to prevent cross coupling therebetween.

I claim:

l. Stereophonic sound signal utilization apparatus including in combination, first and second amplifier channels for conducting respective stereophonic sound signals, a balance potentiometer having a continuous resistance element with the ends thereof connected respectively to said amplifierV channels, said potentiometer further having a fixed center tap and a manually variable tap both connected to a reference point for said apparatus, said variable tap being adjustable over substantially the full length of said resistance element to balance the amplification level of signals in'either of Vsaid channels lwith respect to the other, said fixed vcenter tap having a reduced irnpedance to the reference point with respect to said ,Variable tap thereby improving the electrical separation between said channels and reducing cross coupling of signals therebetween.

2. Stereophonic signal translating apparatus, including in combination, first and second amplifying means for respectively amplifying first and second stereophonic signals, a variable potentiometer control forrbalancing Vthe respective output signals from said first and second amplifying means and including a resistance element, said first and second amplifying means having output circuits connected to opposing terminals on said resistance element, said potentiometer further including a variable tap and a fixed tap,.said variable tap being movable over the full length of said resistance element between said opposing terminals and connected to a reference potential wherebythe stereophonic signals in said output circuits of said first and second amplifying means are developed across portions of said resistance element according to Y the position of said variable tap, and means coupled to the opposing terminals of said resistance element for utilizing the stereophonic signals developed across the portions of said resistance element, said xed tap being located at the electrical midpoint of said resistance element and connected to the reference potential thereby improving electrical separation between said vfirst and second amplifying means to reduce cross-coupling Vbetween first and second stereophonic signals.

3. stereophonic signal translating apparatus, including in combination, first and second amplifying means for amplifying respective stereophonic signals, said Vamplifying means each including first and second electron discharge devices having input, output and common electrodes, said first electron discharge devices each having a series resistance and capacitance means coupled to said output electrodes, said second electron discharge devices each having an input circuit network coupled to said input electrodes, said series resistance and capacitance meansbeing connected to common junctions with said input circuit networks, a variable potentiometer control for balancing the respective output signals from said first and second amplifying means, said balance potentiometer having a resistance element with the ends thereof connected respectively to said common junctions between said series resistance and capacitance means and said input circuit networks, means for respectively applying stereophonic signals to said input electrodes of said rst electron discharge devices, said potentiometer further including a manually variable tap and a xed center tap, said variable tap being movable over the full length of said resistance element between the ends thereof and connected to a reference potential whereby the stereophonic signals from the output of one of said first electron discharge devices are developed across said series resistance means associated therewith and a portion of said resistance element according to the position of said variable tap, and means coupled to said output electrodes of said second electron discharge devices for utilizing said respective stereophonic signals, said xed center tap being located at substantially the electrical midpoint of said resistance element and connected to said reference potential whereby the stereophonic signals from the output of the other of said first electron discharge devices are developed across said series resistance means associated therewith a balance potentiometer having a resistance element of a given length and width with end portions thereof connected respectively to said ampliier channels, said potentiometer further having a fixed tap and a manually variable tap formed by rst and second wipers on a rotatable member, said variable tap being Connected to a reference point for said apparatus so that rotatable adjustment of said variable tap may balance the amplification level of signals in one of said channels with respect to the other, said iixed tap being located on one edge of said resistance element at approximately the electrical midpoint and having a width substantially the same as the width of said resistance element, said fixed tap being connected to said reference point whereby a reduced impedance is effected to said reference point with respect to said variable tap thereby improving the electrical separation between said channels and minimizing cross coupling of signals therebetween.

References Cited in the tile of this patent UNITED STATES PATENTS 2,813,157 Doba Nov. 12, 1967 2,898,410 Brooks Aug. 4, 1959 2,980,766 Nulton et al. Apr. 18, 1961 3,001,019 Uecke et al. Sept. 19, 1961 3,026,378 Fine et al. Mar. 20, 1962 

1. STEREOPHONIC SOUND SIGNAL UTILIZATION APPARATUS INCLUDING IN COMBINATION, FIRST AND SECOND AMPLIFIER CHANNELS FOR CONDUCTING RESPECTIVE STEREOPHONIC SOUND SIGNALS, A BALANCE POTENTIOMETER HAVING A CONTINUOUS RESISTANCE ELEMENT WITH THE ENDS THEREOF CONNECTED RESPECTIVELY TO SAID AMPLIFIER CHANNELS, SAID POTENTIOMETER FURTHER HAVING A FIXED CENTER TAP AND A MANUALLY VARIABLE TAP BOTH CONNECTED TO A REFERENCE POINT FOR SAID APPARATUS, SAID VARIABLE TAP BEING ADJUSTABLE OVER SUBSTANTIALLY THE FULL LENGTH OF SAID RESISTANCE ELEMENT TO BALANCE THE AMPLIFICATION LEVEL OF SIGNALS IN EITHER OF SAID CHANNELS WITH RESPECT 